A fuel cell stack has a structure in which fuel cell (single cell) are stacked. A single cell includes a membrane electrode assembly (hereinafter, referred to as “MEA”) having a polymer electrolyte membrane and a pair of catalyst electrodes sandwiching the polymer electrolyte membrane; and a pair of separators sandwiching the membrane electrode assembly.
The polymer electrolyte membrane is composed of an electrolyte having an ion exchange membrane of a fluorine resin type having a sulfonic acid group, or a polymer ion exchange membrane such as an ion exchange membrane of a hydrocarbon resin.
The catalyst electrode includes a catalyst layer which contacts with the polymer electrolyte membrane for promoting an oxidation-reduction reaction in the catalyst electrode, and a gas diffusion layer located at the outside of the catalyst layer and that has permeability and conductivity. The gas diffusion layer includes a carbon coat layer for enhancing contact property to the catalyst layer, which contacts with the catalyst layer, and a gas diffusion base layer for diffusing an externally supplied gas so as to allow the gas to be supplied to the catalyst layer. The catalyst layer of a fuel electrode layer contains platinum or an alloy of platinum and ruthenium, for example, and the catalyst layer of an air electrode contains platinum or an alloy of platinum and cobalt, for example.
The separator is a conductive member for preventing a fuel gas supplied to the fuel electrode and an oxide gas supplied to the air electrode from being mixed with each other.
Since the fuel cell stack has the single cells stacked, it can electrically be connected in serial. The fuel cell stack also has a pair of end plates that sandwiches the cell stack body (e.g., refer to Patent Document 1). The end plate has a fluid tube body for supplying a gas or a cooling medium to the fuel cell stack or for discharging the gas or the cooling medium from the fuel cell stack.
FIG. 1 is a sectional view illustrating fuel cell stack 10 disclosed in Patent Document 1. As illustrated in FIG. 1, the fuel cell stack 10 disclosed in Patent Document 1 has cell stack body 11, and end plates 15 and 17 that hold the cell stack body 11. The end plate 13 has fluid tube bodies 17 and 19.
In fuel cell stack 10 disclosed in Patent Document 1, end plate 13 and fluid tube bodies 17 and 19 are in contact with each other without having a gap therebetween as illustrated in FIG. 1. There has also been known a technique in which the fluid tube body is made of a material having a low thermal conductivity in order to prevent a heat of a fluid flowing through the fluid tube body from transferring to the end plate (e.g., see Patent Document 2).
When a fuel gas (containing hydrogen) and an oxide gas (containing oxygen) are supplied to the respective single cells in the fuel cell stack having the above-mentioned configuration, electric energy can continuously be taken out. A chemical reaction generated due to the supply of the fuel gas and the oxide gas to the single cell will be described below.
A hydrogen molecule supplied to the fuel electrode is divided into a hydrogen ion and an electron by the catalyst layer of the fuel electrode. The hydrogen ion moves toward the air electrode through the humidified polymer electrolyte membrane. On the other hand, the electron moves toward the air electrode to which the oxide gas is supplied through an external circuit. In this case, the electron passing through the external circuit can be used as electric energy. At the catalyst layer of the air electrode, the hydrogen ion moving through the polymer electrolyte membrane, the electron moving through the external circuit, and the oxygen supplied to the air electrode react with one another to produce water. The above-mentioned chemical reaction also produces heat.
When the fuel gas and the oxide gas are supplied to the fuel cell as described above, the electric energy and thermal energy can simultaneously be obtained. Therefore, the fuel cell stack is utilized as a home cogeneration system for power generation and hot-water supply (e.g., see Patent Document 3). In the home cogeneration system, the heat generated during the power generation is successively collected by using a cooling medium discharged from the fluid tube body. The collected heat is stored in a hot-water tank, and utilized as necessary likewise the electric energy.
There has been known a fuel cell system having the fuel cell stack sandwiched between the end plates, a fuel processing apparatus that produces a fuel gas supplied to the fuel cell stack, and an interconnect section that connects the end plates and the fuel processing apparatus (e.g., see Patent Document 4). In the fuel cell system disclosed in Patent Document 4, a gap is partially formed between the interconnect section and the end plates in order to minimize the transfer of heat between the fuel processing apparatus and the fuel cell stack.    Patent Document 1: Japanese Patent Application Laid-Open No. 2007-294330    Patent Document 2: Japanese Patent Application Laid-Open No. H09-063623    Patent Document 3: Japanese Patent Application Laid-Open No. 2008-293996    Patent Document 4: U.S. Patent Application Publication No. 2006/0134470